Transmission system transmitting information using signals with a non-constant envelope may encounter signal configurations that have a high crest factor or peak to average power ratio (PAPR). A high crest factor or PAPR negatively affects the efficiency of power amplifiers in the transmitter chain. Current wireless communications systems, for example, favour multicarrier modulation formats due to their high spectral efficiency and robustness in frequency selective channels. One example of a popular multicarrier modulation format is OFDM. The problem of high crest factor or PAPR can be particularly pronounced for multicarrier signals and specifically for OFDM signals. Several crest-factor reduction (CFR) methods exist which try to mitigate this problem.
FIG. 1 shows the block diagram of a typical OFDM transmitter employing crest factor reduction. Initially, input data 10 is mapped to a set of symbols which are allocated to N subcarriers in a bit to symbol mapping unit 20. Thereafter, an IFFT operation 30 provides the time-domain signal to be transmitted. Typically, an oversampled IFFT is used in this step. The oversampling factor is denoted as L. Many Crest Factor Reduction (CFR) methods involve an initial clipping stage by which the input signal amplitude is clipped whenever it crosses a predetermined threshold above the average power. This clipping stage is identified by reference numeral 40 in FIG. 1. The clipping stage 40 directly sets the crest factor of the signal to the desired value. However, it also introduces both in-band and out-of-band noise. The spectral density of this noise depends on the clipping threshold.
Many communications systems have very strict requirements in terms of maximum error vector magnitude (EVM) or adjacent channel leakage ratio (ACLR). To meet these requirements the clipped signal is processed further in known crest factor reduction methods in order to limit the amount of noise introduced by the clipping as well as the EVM and ACLR to acceptable levels.
In the prior art system shown in FIG. 1, the clipped time domain signal is transformed back into the frequency domain using a FFT operation 50. In the frequency domain, out-of-band noise can easily be attenuated and in-band distortion reduced in a frequency domain processing unit 60, to meet the regulatory levels or and/or operator requirements. Thereafter, a conversion back to the time domain is performed via an IFFT operation 70.
Other methods use a simpler clipping and filtering approach. Such a method is depicted in FIG. 2. In this method an input signal 100 labelled x is clipped in a clipping unit 110 to create a clipped signal 120, labelled xc. This clipped signal 120 is subtracted from the original input signal 100 in a subtractor 130 to form a signal 140, labelled xD), that contains only the above-threshold peaks. The peak signal 140 is then filtered in a filter 150 to attenuate the out-of-band noise, creating a filtered signal 160. The filtered signal 160 is then subtracted from the original input signal 100.
Although this approach is simple, it offers little control over distortion and spectral mask. Moreover, the use of traditional filters may not be practical when transmission takes place over many non-contiguous subcarriers, as is the case in OFDMA mobile terminals. Filtering non-contiguous subcarriers would require many closely-spaced notches in the filter's frequency response. This requirement can increase the order of the filter beyond practical values or requiring multiple filters. A similar problem occurs in base stations if multiple bands, as occur in LTE and WCDMA systems, have to be transmitted simultaneously by the same RF amplifier, so that crest factor reduction has to be performed on the aggregate signal.